This invention relates to a dielectric waveguide for the transmission of electromagnetic waves. More particularly, the invention relates to a dielectric waveguide having higher order mode suppression filters.
Electromagnetic fields are characterized by the presence of an electric field vector E orthogonal to a magnetic field vector H. The oscillation of these components produces a resultant wave which travels in free space at the velocity of light and is transverse to both of these vectors. The power magnitude and direction of this wave is obtained from the Poynting vector given by: EQU P=E.times.H(Watts/m.sup.2)
Electromagnetic waves may exist in both unbounded media (free space) and bounded media (coaxial cable, waveguide, etc.). This invention relates to the behavior of electromagnetic energy in a bounded medium and, in particular, in a dielectric waveguide.
For propagation of electromagnetic energy to take place in a bounded medium, it is necessary that Maxwell's Equations are satisfied when the appropriate boundary conditions are employed.
In a conventional metal waveguide, these conditions are that the tangential component of the electric field, E.sub.t, is zero at the metal boundary and also that the normal component of the magnetic flux density, B.sub.n, is zero.
The behavior of such a waveguide structure is well understood. Under excitation from external frequency sources, characteristic field distributions or modes will be set-up. These modes can be controlled by variation of frequency, waveguide shape and/or size. For regular shapes, such as rectangles, squares or circles, the well-defined boundary conditions mean that operation over a specific frequency band using a specific mode is guaranteed. This is the case with most rectangular waveguide systems operating in a pure TE.sub.10 mode. This is known as the dominant mode in that it is the first mode to be encountered as the frequency is increased. The TE.sub.mn type nomenclature designates the number of half sinusoidal field variations along the x and y axes, respecitvely.
Another family of modes in standard rectangular waveguides are the TM.sub.mn modes, which are treated in the same way. They are differentiated by the fact that TE.sub.mn modes have no E.sub.z component, while TM.sub.mn modes have no H.sub.z component.
The dielectric waveguide disclosed in U.S. Pat. No. 4,463,329 does not have such well-defined boundary conditions. In such a dielectric waveguide, fields will exist in the polytetrafluoroethylene (PTFE) cladding medium. Their magnitude will decay exponentially as a function of distance away from the core medium. This phenomena also means that, unlike conventional waveguides, numerous modes may, to some degree, be supported in the waveguide depending upon the difference in dielectric constant between the mediums, the frequency of operation and the physical dimensions involved. The presence of these so-called "higher order" modes is undersirable in that they extract energy away from the dominant mode, causing excess loss. They cause, in certain cases, severe amplitude ripple and they contribute to poor phase stability under conditions of flexure.
A launching horn employed in conjunction with a waveguide taper performs a complex impedance transformation from conventional waveguide to the dielectric waveguide. Techniques such as the finite element method may be used to make this transformation as efficient as possible. However, the presence of any impedance discontinuity will result in the excitation of higher order modes.
Having described the ways in which higher order modes may be stimulated in such a dielectric waveguide assembly, mode filters for suppressing their presence will now be disclosed.